This invention relates to a process for treating municipal solid waste material and more particularly to such a process which is adapted to separate and classify municipal solid waste into various inorganic, organic and synthetic fractions thereof. The major portion of the inorganic fraction consists of metal and glass containers while the remainder consists of bits of masonry, ceramics, automobile parts, building materials and the like. The synthetic fraction usually consists of plastic containers, plastic film, toys, toothbrushes and the like in addition to a variety of materials such as articles of clothing. The organic fraction usually represents more than 80% by weight of the municipal solid waste and consists largely of ligno-cellulose, such as paper products, with the remainder consisting of yard waste, food scraps, dead animals and the like.
The organic fraction of municipal solid waste represents the industrial world's largest economically assessible source of ligno-cellulose feed stock for conversion to alcohol and other industrial chemicals. At the same time, municipal solid waste is an environmental concern to which the industrial world is attempting to find a solution due to the depletion of natural resources and the dwindling availability of land fill areas.
Heretofore, the most common method for classifying municipal solid waste materials has been by air classification wherein the waste materials are first shredded or ground for particle size reduction. This material is then subjected to currents of upwardly flowing air whereby the lighter fraction thereof tends to float as the heavier materials sink. The major portion of this lighter fraction consists of paper, plastic film, fabric and the like while the heavier fraction consists of wet paper, metals, glass, plastic, fruit, vegetables, meats, wood and the like. This method requires considerable time and energy in the shredding of the municipal solid waste materials. Also, bits of glass and metal are embedded in the lighter fraction since the waste material has not been properly classified into metal, glass, plastic, paper, food scraps and the like.
Another method heretofore employed utilizes the paper industry's technique for pulping. In this method, municipal solid waste is subjected to size reduction and is then placed in large vats where the organic fraction is pulped into a slurry of approximately 3% solids. The heavier metals, glass, masonry and the like may then be removed by settling or centrifuging. This method is also energy intensive in that large quantities of water are required in order to produce a slurry of 3% solids. This excess water must then be removed, such as by de-watering the slurry to a cake of less than 50% moisture to produce a product which may then be used as a fuel.
The Grube U.S. Pat. No. 4,050,899, dated Sept. 27, 1977, discloses a method for fragmenting solid waste wherein the waste material is dehydrated, comminuted and expelled into an open ended composting pit.
In my prior U.S. Pat. No. 4,342,830, dated Aug. 3, 1982, steam pressure separation is disclosed wherein softened organics are forced through perforations upon sudden release of pressure whereby inorganics, such as cans, bottles and the like are left behind. This method is somewhat more energy intensive than some of the methods heretofore employed to classify municipal solid waste in that the steam used to force the softened organics out is difficult to recover.